Use mimeType from metadata instead of blindly re-determining it during uploads

Summary:
LibKGAPI uses a files name and data when determining content type. If the File upload mime-type is specified, don't re-determine it.
This avoids having to expose the upload filename while using temporary files.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D22787